Combustion of fossil fuels (e.g., in a furnace) produces flue gases containing nitrogen oxides (NOx), which can be harmful to both humans and the environment. It is therefore desirable to remove NOx from flue gases prior to the flue gases entering the atmosphere.
Various techniques have been developed to control emissions of NOx. Typically, NOx is reacted to form nitrogen and water using a selective catalytic reduction (SCR) technique in which NOx reacts with a reducing agent in the presence of a catalyst. In the prior art, ammonia (in a gaseous state, in aqueous solution, or by conversion of compounds such as urea) is often used as a reducing agent in conjunction with a catalyst, such as a tungsten vanadia-titania catalyst. Such SCR systems, however, require ammonia to be transported and stored on site, which can present safety concerns and subject a plant to additional regulations. Further, such SCR systems typically require higher operating temperatures, e.g., 250-500 degrees Celsius (482-932 degrees Fahrenheit), which can necessitate costly equipment modifications to accommodate the temperature required by the SCR systems. For example, a typical flue gas boiler used to recover steam in the convection section of a furnace can be located at a position where the temperature range is similar to that required by the SCR system, e.g., 220-700 degrees Celsius (428-1,292 degrees Fahrenheit). Thus, to recover the same amount of steam in the convection section, the flue gas boiler equipment would typically have to be split.
Other prior art SCR systems use hydrogen as a reducing agent in conjunction with catalysts that comprise a noble metal such as platinum and palladium on a substrate. These SCR systems obviate the need to store ammonia on site and can also operate at lower temperatures (e.g., 100-400 degrees Celsius or 212-752 degrees Fahrenheit). However, the catalysts that comprise noble metals that are used in these systems are costly.
Thus, there is a need in the art for improved SCR systems and processes which use hydrogen as a reducing agent, thereby obviating the need for ammonia and allowing operation at relatively low temperatures, but utilize a less-costly catalyst.